The present invention generally relates to probe systems used in integrated circuit testing stations, and more particularly to a piezoelectric pressure sensing apparatus for determining the amount of pressure exerted on an integrated circuit test probe during use.
An important step in the production of integrated circuits involves the testing of each circuit to determine whether it has been properly manufactured. It is generally desirable to evaluate the performance of integrated circuits as early as possible in the fabrication process. To accomplish this, electrical connections are made to all of the integrated circuit's external connection points. Test signals are then applied to the circuit, and its performance is evaluated.
To test integrated circuits in an efficient manner, an integrated circuit test probe system has been developed by Tektronix, Inc. of Beaverton, Oreg. which is the subject of co-pending patent applications. This system consists of a flexible probe in the form of a small square of transparent polyimide film approximately 0.001" thick. The underside of the film includes a plurality of metal pads deposited thereon. The metal pads are preferably manufactured from nickel, and are arranged in a pattern which matches the contact areas (bond pads) on the integrated circuit. The metal pads on the polyimide film probe are electrically connected by transmission lines routed to the edges of the film probe using, for example, microstrip line geometry techniques.
In a preferred form of the probe system, a ground plane is deposited on the top surface of the polyimide film. However, the ground plane does not cover the areas of the film in the vicinity of the metal pads. As a result, the metal pads can be viewed through the film, thereby permitting visual alignment of the pads with respect to the circuit being tested.
The polyimide film probe is mounted to and supported along its edges by a printed circuit board. Transmission lines on the printed circuit board provide a connection between the probe and coaxial connectors along the outside periphery of the printed circuit board.
To use the above-described testing system, the polyimide film probe and printed circuit board are mounted in a fixed position on a support structure. Beneath the probe and support structure, an apparatus is provided which includes means for elevating the circuit to be tested upward toward the probe. Preferably, a vacuum chuck device is used which includes a platform having a vacuum system for maintaining an integrated circuit chip or wafer thereon, and a stepper motor for progressively elevating the platform and circuit upward toward the probe. To test the integrated circuit, the circuit must be raised upward in an amount sufficient to contact the metal pads on the underside of the probe.
However, it is necessary to monitor the contact pressure between the circuit and probe so that such pressure can be determined and repeated in consecutive tests. Repeated testing of the circuit is necessary in order to obtain an accurate measurement of the performance capability of the circuit. In addition, the contact pressure must be monitored to ensure that the circuit will not exert undue pressure on the probe when the metal pads of the probe contact the circuit. If the stepper motor in the vacuum chuck is not precisely controlled, excessive pressure will be generated by the upward movement of the integrated circuit against the polyimide film probe. Such pressure can cause significant damage to the probe. Typically, a pressure level of 3 grams exerted on each probe pad will be sufficient to enable proper testing of the circuit in a non-destructive manner. Pressure levels exceeding 10 grams per probe pad are likely to cause damage. However, this value may vary, depending on the type and thickness of the probe being used.
The present invention represents a probe station accessory for sensing the amount of pressure exerted on a test probe by an upwardly-moving integrated circuit. As a result, the amount of pressure being applied can be accurately determined.